It's hard to know. Did she say she heard your son say this? Did she say anything to your child at the time? Or did she say her child reported this to her? Probably the latter. So you don't know.
It is routine for kids to say stuff like this and have no memory of it - it was an automatic response, and to them it's often like saying, "Hi" or "It's sunny today" - not memorable. But, there are others who deny it because they know you're upset about it. It could also be that her son didn't want to play outside or just wanted an excuse to watch TV, so he blamed something on your child. At 8 and 7, you don't know. I don't think you have to get into the "not my child" thing, even though you already doing it.
So I would say you spoke to your child about it and in general about the importance of being kind, and leave it there. I might make a statement about parents always having to remind kids this age of what things sound like to other people, and how it's always a balancing act to figure out how often and how deeply to get involved. What I would not do is get all revved up about what to say if she says X, vs. what to say if she says Y or Z. There is no nice way to say, "You know, Jennifer, I listened to you and then I listened to my son, and I believe my son."
Moreover, too much "what if" beforehand will make any response on your part seem rehearsed, and will make you more tense in her presence (which may make her think you are defensive).
Beyond that, it's important to realize that we need to raise our children to not fall apart every time someone says something thoughtless, hurtful or downright mean. We cannot police every child (or every adult, for that matter) and intervene in every interaction or relationship our kids have. We can't put a bandaid on every boo-boo - some just have to scab over and heal in time. If neighborhood kids can't negotiate their differences and find a way to play together, then they have to stay home, because Mom isn't setting up daily playdates with "acceptable" friends. So I suggest you take the long view, encourage your son to do so as well (while being kind and thoughtful), and (gently) encourage your neighbor to do the same.
I had a pretty good network of friends and neighbors, and my son knew that there was "community parenting" going on - for good and for bad. There were over a dozen homes where he could go to use a bathroom or get a scraped knee cleaned up, but also a dozen families who would let me know if he did a good thing or did a crappy thing. Rollerblading downtown without a helmet on? I knew about it before he got home. He learned a lot that day - that people care about his safety, and know my values.